Add offset_of! macro (RFC 3308)

Implements rust-lang/rfcs#3308 (tracking issue rust-lang#106655) by adding the built in macro `core::mem::offset_of`. Two of the future possibilities are also implemented:

* Nested field accesses (without array indexing)
* DST support (for `Sized` fields)

I wrote this a few months ago, before the RFC merged. Now that it's merged, I decided to rebase and finish it.

cc `@thomcc` (RFC author)

Add offset_of! macro (RFC 3308)

Implements rust-lang/rfcs#3308 (tracking issue rust-lang#106655) by adding the built in macro `core::mem::offset_of`. Two of the future possibilities are also implemented:

* Nested field accesses (without array indexing)
* DST support (for `Sized` fields)

I wrote this a few months ago, before the RFC merged. Now that it's merged, I decided to rebase and finish it.

cc `@thomcc` (RFC author)

This macro is used to compute the offset of a field in a struct.

This commit enables two unstable features that are necessary for using
the macro in a constant. However, this is not a problem as the macro
will become available from the Rust standard library soon [1]. The
unstable features can be disabled again once that happens.

The macro in this patch does not support sub-fields. That is, you cannot
write `offset_of!(MyStruct, field.sub_field)` to get the offset of
`sub_field` with `field`'s type being a struct with a field called
`sub_field`. This is because `field` might be a `Box<SubStruct>`, which
means that you would be trying to compute the offset to something in an
entirely different allocation. There's no easy way to fix the current
macro to support subfields, but the version being added to the standard
library should support it, so the limitation is temporary and not a big
deal.

Link: rust-lang/rust#106655 [1]
Signed-off-by: Wedson Almeida Filho <[email protected]>
Co-developed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>

Add offset_of! macro (RFC 3308)

Implements rust-lang/rfcs#3308 (tracking issue rust-lang#106655) by adding the built in macro `core::mem::offset_of`. Two of the future possibilities are also implemented:

* Nested field accesses (without array indexing)
* DST support (for `Sized` fields)

I wrote this a few months ago, before the RFC merged. Now that it's merged, I decided to rebase and finish it.

cc `@thomcc` (RFC author)

Implement builtin # syntax and use it for offset_of!(...)

Add `builtin #` syntax to the parser, as well as a generic infrastructure to support both item and expression position builtin syntaxes. The PR also uses this infrastructure for the implementation of the `offset_of!` macro, added by rust-lang#106934.

cc `@petrochenkov` `@DrMeepster`

cc rust-lang#110680 `builtin #` tracking issue
cc rust-lang#106655 `offset_of!` tracking issue

This macro is used to compute the offset of a field in a struct.

This commit enables two unstable features that are necessary for using
the macro in a constant. However, this is not a problem as the macro
will become available from the Rust standard library soon [1]. The
unstable features can be disabled again once that happens.

The macro in this patch does not support sub-fields. That is, you cannot
write `offset_of!(MyStruct, field.sub_field)` to get the offset of
`sub_field` with `field`'s type being a struct with a field called
`sub_field`. This is because `field` might be a `Box<SubStruct>`, which
means that you would be trying to compute the offset to something in an
entirely different allocation. There's no easy way to fix the current
macro to support subfields, but the version being added to the standard
library should support it, so the limitation is temporary and not a big
deal.

Link: rust-lang/rust#106655 [1]
Signed-off-by: Wedson Almeida Filho <[email protected]>
Co-developed-by: Alice Ryhl <[email protected]>
Signed-off-by: Alice Ryhl <[email protected]>

This macro is used to compute the offset of a field in a struct.

This commit enables two unstable features that are necessary for using
the macro in a constant. However, this is not a problem as the macro
will become available from the Rust standard library soon [1]. The
unstable features can be disabled again once that happens.

The macro in this patch does not support sub-fields. That is, you cannot
write `offset_of!(MyStruct, field.sub_field)` to get the offset of
`sub_field` with `field`'s type being a struct with a field called
`sub_field`. This is because `field` might be a `Box<SubStruct>`, which
means that you would be trying to compute the offset to something in an
entirely different allocation. There's no easy way to fix the current
macro to support subfields, but the version being added to the standard
library should support it, so the limitation is temporary and not a big
deal.

Link: rust-lang/rust#106655 [1]
Signed-off-by: Alice Ryhl <[email protected]>
Co-Developed-by: Wedson Almeida Filho <[email protected]>
Signed-off-by: Wedson Almeida Filho <[email protected]>

Make offset_of field parsing use metavariable which handles any spacing

As discussed at and around comments rust-lang#106655 (comment) and rust-lang#106655 (comment), the current arguments to offset_of do not accept all the whitespace combinations: `0. 1.1.1` and `0.1.1. 1` are currently treated specially in `tests/ui/offset-of/offset-of-tuple-nested.rs`.

They also do not allow [forwarding individual fields as in](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=444cdf0ec02b99e8fd5fd8d8ecb312ca)
```rust
macro_rules! off {
    ($a:expr) => {
        offset_of!(m::S, 0. $a)
    }
}
```

This PR replaces the macro arguments with `($Container:ty, $($fields:expr)+ $(,)?)` which does allow any arrangement of whitespace that I could come up with and the forwarding of fields example above.

This also allows for array indexing in the future, which I think is the last future extension to the syntax suggested in the offset_of RFC.

Tracking issue for offset_of: rust-lang#106655
`@rustbot` label F-offset_of

`@est31`

Make offset_of field parsing use metavariable which handles any spacing

As discussed at and around comments rust-lang#106655 (comment) and rust-lang#106655 (comment), the current arguments to offset_of do not accept all the whitespace combinations: `0. 1.1.1` and `0.1.1. 1` are currently treated specially in `tests/ui/offset-of/offset-of-tuple-nested.rs`.

They also do not allow [forwarding individual fields as in](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=444cdf0ec02b99e8fd5fd8d8ecb312ca)
```rust
macro_rules! off {
    ($a:expr) => {
        offset_of!(m::S, 0. $a)
    }
}
```

This PR replaces the macro arguments with `($Container:ty, $($fields:expr)+ $(,)?)` which does allow any arrangement of whitespace that I could come up with and the forwarding of fields example above.

This also allows for array indexing in the future, which I think is the last future extension to the syntax suggested in the offset_of RFC.

Tracking issue for offset_of: rust-lang#106655
``@rustbot`` label F-offset_of

``@est31``

Rollup merge of rust-lang#119532 - GKFX:offset-of-parse-expr, r=est31

Make offset_of field parsing use metavariable which handles any spacing

As discussed at and around comments rust-lang#106655 (comment) and rust-lang#106655 (comment), the current arguments to offset_of do not accept all the whitespace combinations: `0. 1.1.1` and `0.1.1. 1` are currently treated specially in `tests/ui/offset-of/offset-of-tuple-nested.rs`.

They also do not allow [forwarding individual fields as in](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=444cdf0ec02b99e8fd5fd8d8ecb312ca)
```rust
macro_rules! off {
    ($a:expr) => {
        offset_of!(m::S, 0. $a)
    }
}
```

This PR replaces the macro arguments with `($Container:ty, $($fields:expr)+ $(,)?)` which does allow any arrangement of whitespace that I could come up with and the forwarding of fields example above.

This also allows for array indexing in the future, which I think is the last future extension to the syntax suggested in the offset_of RFC.

Tracking issue for offset_of: rust-lang#106655
``@rustbot`` label F-offset_of

``@est31``

Make offset_of field parsing use metavariable which handles any spacing

As discussed at and around comments rust-lang/rust#106655 (comment) and rust-lang/rust#106655 (comment), the current arguments to offset_of do not accept all the whitespace combinations: `0. 1.1.1` and `0.1.1. 1` are currently treated specially in `tests/ui/offset-of/offset-of-tuple-nested.rs`.

They also do not allow [forwarding individual fields as in](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=444cdf0ec02b99e8fd5fd8d8ecb312ca)
```rust
macro_rules! off {
    ($a:expr) => {
        offset_of!(m::S, 0. $a)
    }
}
```

This PR replaces the macro arguments with `($Container:ty, $($fields:expr)+ $(,)?)` which does allow any arrangement of whitespace that I could come up with and the forwarding of fields example above.

This also allows for array indexing in the future, which I think is the last future extension to the syntax suggested in the offset_of RFC.

Tracking issue for offset_of: #106655
``@rustbot`` label F-offset_of

``@est31``

…wesleywiser

Stabilize single-field offset_of

This PR stabilizes offset_of for a single field. There has been some further discussion at rust-lang#106655 about whether this is advisable; I'm opening the PR anyway so that the code is available.

Rollup merge of rust-lang#118799 - GKFX:stabilize-simple-offsetof, r=wesleywiser

Stabilize single-field offset_of

This PR stabilizes offset_of for a single field. There has been some further discussion at rust-lang#106655 about whether this is advisable; I'm opening the PR anyway so that the code is available.

Stabilize single-field offset_of

This PR stabilizes offset_of for a single field. There has been some further discussion at rust-lang/rust#106655 about whether this is advisable; I'm opening the PR anyway so that the code is available.

Feature gate enums in offset_of

As requested at rust-lang/rust#106655 (comment), put enums in offset_of behind their own feature gate.

`@rustbot` label F-offset_of

Remove option_payload_ptr; redundant to offset_of

The `option_payload_ptr` intrinsic is no longer required as `offset_of` supports traversing enums (#114208). This PR removes it in order to dogfood offset_of (as suggested at rust-lang/rust#106655 (comment)). However, it will not build until those changes reach beta (which I think is within the next 8 days?) so I've opened it as a draft.

Feature gate enums in offset_of

As requested at rust-lang/rust#106655 (comment), put enums in offset_of behind their own feature gate.

`@rustbot` label F-offset_of

Remove option_payload_ptr; redundant to offset_of

The `option_payload_ptr` intrinsic is no longer required as `offset_of` supports traversing enums (#114208). This PR removes it in order to dogfood offset_of (as suggested at rust-lang/rust#106655 (comment)). However, it will not build until those changes reach beta (which I think is within the next 8 days?) so I've opened it as a draft.